Nickel-based and cobalt-based superalloy materials are commonly used to provide high mechanical strength for very high temperature applications, such as for the hot gas path components of a gas turbine engine. The design firing temperatures of modern gas turbines continue to increase in order to improve the efficiency of such engines, and new superalloy compositions and processing methods are developed to accommodate these higher temperatures.
The term “superalloy” is used herein as it is commonly used in the art; i.e., a highly corrosion and oxidation resistant alloy that exhibits excellent mechanical strength and resistance to creep at high temperatures. Superalloys typically include a high nickel or cobalt content. Examples of superalloys include alloys sold under the trademarks and brand names Hastelloy, Inconel alloys (e.g. IN 738, IN 792, IN 939), Rene alloys (e.g. Rene N5, Rene 80, Rene 142), Haynes alloys, Mar M, CM 247, CM 247 LC, C263, 718, X-750, ECY 768, 282, X45, PWA 1483 and CMSX (e.g. CMSX-4) single crystal alloys.
Efforts have been made to use additive manufacturing techniques for manufacture as well as repair of superalloy components. In additive manufacturing techniques, a heat source is used to melt a specified amount of metal, which is usually in the form of a powder or wire, onto a base material. By repeating the process and controlling the region of deposition of each layer, layers of melted metallic materials are arranged upon preceding layers, resulting in the formation of a desired component. Additive manufacturing (AM) techniques include selective laser melting (SLM), selective laser sintering (SLS), electron beam melting (EBM), laser metal forming (LMF), laser engineered net shape (LENS), and direct metal deposition (DMD). In the SLM technique, a laser beam scans a layer of powder, thereby melting and solidifying the powder in the areas of contact with the laser beam. Benefits to laser processing are its speed, flexibility, cost, and lead-time reduction potential for manufacturing and reconditioning of components used in today's heavy-duty gas turbines.
The assignee of the present invention produces gas turbine engines utilizing a variety of materials, including blades formed of cast nickel-based superalloy material sold by Cannon-Muskegon Corporation under the designation CM-247 LC. CM-247 LC may have an aluminum content exceeding 5.5%, and is known to have the following nominal composition, expressed as weight percentages: carbon 0.07%; chrome 8%; cobalt 9%; molybdenum 0.5%; tungsten 10%; tantalum 3.2%; titanium 0.8%; aluminum 5.6%; boron 0.015%; zirconium 0.01%; hafnium 1.4%; and the balance nickel.
Elemental additions in nickel base superalloys can be broadly characterized as either gamma formers or gamma prime formers. Gamma formers are for example elements found in Group V, VI, and VII, such as Co, Cr, Mo, W, and Fe. The atomic diameters of these alloys are typically 3-13% different than Ni (the primary matrix element). Gamma prime formers are, for example, elements of Group III, IV, and V such as Al, Ti, Nb, Ta, Hf. The atomic diameters of these elements typically differ from Ni by 6-18%.
Nickel-based alloys can be either solid solution or precipitation strengthened. Previously, solid solution strengthened alloys, such as Hastelloy X, were used in applications requiring only modest strength, while precipitation strengthened alloys have been required for extreme temperature applications, such as hot sections of a gas turbine engine.
Laser additive manufacturing of some high aluminum and/or titanium nickel base alloys is limited. The gamma prime phase in a nickel-aluminum alloy is referred to as Ni3Al or Ni3 [Al, Ti]. Modern nickel base gamma prime hardened superalloys (“precipitation strengthened” superalloys) may have a volume fraction of 70% gamma prime phase to gamma phase. The gamma (disordered) phase forms a matrix through which the gamma prime (ordered) phase precipitates. As used herein, the term “gamma/gamma prime superalloy” refers to an alloy having a gamma phase matrix in which gamma prime precipitates are embedded. The ordered gamma prime phase present within the gamma matrix acts as a barrier to dislocation motion, thereby strengthening the material. For this reason, this gamma prime phase, when present in high volume fractions, drastically increases the strength of these alloys, and are thus desirable.
Unfortunately, efforts to manufacture workable precipitation strengthened superalloys using laser additive manufacturing have been limited due to incipient cracking which occurs when the superalloy is subjected to laser heating, partial melt, and subsequent rapid cooling. CM-247 LC powder forms gamma/gamma prime superalloy upon cooling from laser heating, but the percentage of gamma prime precipitate that is formed is too high. This results in incipient melting resulting in cracking during cooling, which renders the formed superalloy unsuitable for extreme environments and applications.
Previous work by the present inventors (cited in the inventors' previous United States Patent Application Publication No. US 2015/0050157 A1), including a detailed study of factors affecting weldability of Ni base superalloys and their susceptibility to cracking, has led the present inventors to conclude that a gamma prime phase volume fraction in an amount generally less than about 20-30 wt % is indicative of weldability without unacceptable susceptibility to cracking. A gamma prime content greater than about 60 wt % is generally indicative of nonweldability (that is, susceptibility to strain age cracking) while intermediate gamma prime values typically indicate difficult and expensive welding. Substantially the same conclusions can be drawn for additive or weld build-up processes. That is, a gamma prime phase present in an amount less than about 20-30 wt % is indicative of weld build up without unacceptable susceptibility to cracking. Gamma prime greater than about 60 wt % is generally indicative of weld build up having an unacceptable susceptibility to cracking.